Organic coatings such as those made from radiation curable and thermoset polymers are known for use in applications where the appearance of the coating is fundamental to its overall function, such that scratches on or through the surface would render the coating unsuitable or undesirable for continued use. Scratches not only detract from the overall appearance, but also compromise the integrity of the substrate. Past approaches to scratch resistant coatings focused on the use of harder polymers. However, these harder polymers were a result of increased crosslink density and showed inherent negative properties with respect to shrinkage, adhesion, flexibility, and impact resistance.
U.S. Pat. No. 5,853,809 describes scratch resistant clearcoats containing surface reactive microparticles. The coating composition contains a film forming binder system containing a crosslinkable resin, colorless inorganic microparticles of about 1.0 to about 1000 nanometers having a functionality reactable with the crosslinkable resin, and a solvent system for the crosslinkable resin.
Some attempts have been made in the art to improve abrasion resistance but these efforts have not addressed the issue of scratch resistance. For example, WO 00/39042 describes a surface covering comprising at least one layer containing wear-resistant particles, such as aluminum oxide. The particle size of the wear-resistant particles is from about 10 micrometers to about 350 micrometers, and more preferably from about 20 micrometers to about 250 micrometers, and most preferably from about 30 micrometers to 200 micrometers. Wear resistance is determined by abrasion tests such as the Taber abrasion test and the effect of the particles in the surface coating is described as providing abrasion resistance. No differentiation with respect to scratch resistance is made.
Likewise, EP 235 914 describes coating compositions for producing a texture finish onto a substrate, the composition comprising an adhesion promoter for promoting adhesion to the substrate, a radiation-curable component and a texture modifying amount of microspheres substantially homogeneously dispersed therein. The microspheres can be glass and/or ceramic and/or polymeric materials. The incorporation of fine glass, ceramic or polymeric solid beads or hollow spheres into a suitable radiation-curable component which, on curing, sets to form a matrix holding the beads or spheres on the substrate, enables a textured appearance to be provided and an abrasion resistance comparable to prior art methods. The particle size of the microspheres is up to 120 micrometers and more particularly from 15 to 60 micrometers and advantageously about 30 micrometers.
U.S. Pat. No. 6,399,670 relates to a pre-cured coating mixture comprising a radiation curable resin, an initiator, and texture-producing particles having an effective size to provide a macroscopic texture upon application of the mixture on a substrate. Preferred sizes of the texture-producing particles is 30 to 350 μm. The coating mixture utilizes nanometer sized inorganic particles having diameters from 1 to 100 nm (0.001 micrometers to 0.1 micrometers). Example 3 shows that inorganic particles (alumina) outside of the range of the '670 patent on the order of 1.5 micrometers produce coatings which are not as transparent as coatings of the invention which utilize nano-size inorganic particles as disclosed and that scratch resistance was poor inasmuch as visual scratches were present after testing.
Thus, there have been attempts to provide clear top-coatings having greater abrasion resistance without regard to the scratch resistance of the material. There also have been attempts to find organic coatings with scratch resistance that provide appropriate properties for a variety of uses. However, these attempts have required the use of harder polymers, reactive systems or texture-modifying systems. Thus, there is still a need in the art for organic coatings which provide improved scratch resistance without negatively impacting other physical properties of the coating such as color, flexibility, gloss, gloss retention, impact resistance, opacity, and stain resistance.